It is generally known that process plants, such as refineries, chemical plants or pulp and paper plants, consist of numerous process control loops connected together to produce various products. Flow control valves are disposed in the process control loop, and typically use a control element such as a valve plug, a valve disc, a globe, or other suitable control element, in order to manipulate a fluid flowing through the system, such as gas, steam, water, or a chemical compound.
It is generally understood that various control valve configurations may be specifically applicable for certain applications. For example, when a quick-opening valve with a narrow control range is suitable, a rotary control valve, such as a butterfly valve, may be used. Alternatively, when precise control over a large control range is required, a sliding stem control valve having a valve plug may be used. In any configuration, such control valves are generally coupled to a control device such as an actuator, which controls the exact opening amount of the control valve in response to a control signal. In each case, the valve stem extends into the valve body and is connected to the control element.
In order to prevent or minimize leakage past the valve stem, control valves typically employ valve packing around the valve stem. Such valves often use a packing flange and a packing follower, which apply pressure to the valve packing. The valve packing is typically separated into two distinct packing sets separated by a lantern ring. In such an arrangement, a leak-off port is provided through the valve body to provide flow communication adjacent the lantern ring. Such an arrangement allows personnel to determine whether the packing below the lantern ring has failed.